The present invention relates to a method and an apparatus for deicing objects, and to a method for transmitting overpressure through a passage which is blocked for dynamic pressure.
Rapid tempering has a number of applications. Thus, it is desirable to be able to quickly deice and dry vehicles, such as railway wagons, cars and deck containers. In winter it is also desirable to be able to quickly deice material, such as iron girders, which are stored outdoors, before they are used for instance in a welding shop, like a shipyard or the like. Another field of application is thawing of frozen products.
The object of the present invention is to provide a method and provide an apparatus for deicing objects, which method and apparatus are energy saving and, especially at deicing, do not result in large contents of moisture of the ambient air with associated problems of condensation on cold surfaces.
This object is obtained by a method of deicing objects which is characterized in that air or another gaseous medium having a low overtemperature is blown with high speed onto the object to be tempered, and by an apparatus for deicing objects which is characterized in that means are provided to heat air or another gaseous deicing medium, and in that exhaust means are provided to exhaust said heated medium with high speed in jets directed towards objects to be tempered.
Another object of the invention is to suggest a method of developing a counterpressure to prevent undesired flow of air or diffusion of gas to a room in which a dynamic counterpressure cannot be produced. In this way also the abovementioned problem of condensation can be further reduced by producing such a counterpressure by dry air around those surfaces which are exposed to condensation.
This object is obtained by a method of transmitting an overpressure through a passage which is blocked for dynamic pressure, which method is characterized in that a static pressure is produced across the orifice of said passage.
Deicing of railway wagons has so far been effected only by heat transfer to the underframe or bogie wagon or the train and this has been done by air of high temperature and moderate speed. By means of air heaters equipped with fans, air having a temperature exceeding 50.degree. C., typically 70.degree. C., has been oriented towards parts of the bogies and other places where the quantity of ice usually is most important with a typical striking velocity of 0.5 m/sec.
An obvious disadvantage of this previously known solution is that the high temperature which is used results in evaporation of large quantities of water by the hot air which gives the air high contents of moisture. Evaporation of water consumes large quantities of heat and installed heating effect must be dimensioned for ice melting as well as evaporation of a large quantity of water. The energy consumption therefore becomes unnecessarily large. The moisture absorbed by the air also results in a need for fast circulation of air in the room in which the deicing is performed for the drying of the air.
In the method and apparatus according to the invention the transfer of heat from the air to the bogie wagon of the train is performed by convection.
The rate of transfer which is a function of the coefficient of heat transfer for transmission of heat by convection, .alpha., and the temperature difference between the bogie wagon or wagon chassis and intake air, .DELTA.t, is given by EQU Q=.alpha..multidot..DELTA.t
where Q denotes the transferred quantity of heat.
A lower temperature of the intake air thus can be compensated for by a higher value of .alpha..
The coefficient of heat transfer .alpha. is a function of the radiation temperature, the ambient temperature and the speed of the air towards the surface in question.
The radiation part of the heat transfer is equal to 0.96.multidot. emission number.multidot..DELTA.t.sub.s, where
0.96=Stefan Boltzmann's number, PA1 emission number for water, ice, frost=0.95-0.98, for painted surface=0.90-0.97, PA1 .DELTA.t.sub.s =counterradiation temperature. PA1 .DELTA.t=the temperature difference between air and surface in .degree.C., and PA1 L=characteristic length in meters.
The radiation exchange in deicing of bogie wagons or wagon chassis takes place between the floor of the hall and the bogie wagon or wagon chassis and is influenced only in a marginal manner by flows of air and rates of flows.
The influence on the heat transfer by ambient temperature is according to G Brown for a cold surface: EQU 2.08.multidot..DELTA.t.sup.0.31 .multidot.L.sup.-0.08 k
where
The above temperature difference .DELTA.t can to a great extent be influenced in the present invention since the temperature of the intake air can be chosen.
The relation above between the heat transfer and the temperature difference .DELTA.t is graphically represented in FIG. 1.
The air speed towards the bogie wagon or wagon chassie has an influence on the heat transfer for air speeds exceeding 5 m/sec. This influence can be roughly estimated from the formula EQU .alpha..sub.v =7.6.multidot.v.sup.0.78
where v=air speed in m/sec and is graphically illustrated in FIG. 2.
Like the temperature difference .DELTA.t also the air speed v can be influenced in the apparatus according to the invention in which controlled blowing is used.
From the above relationship and the graphical illustration in FIGS. 1 and 2 it appears that a change in the speed of the air of 0.5 m/sec gives roughly the same change of the .alpha.-value as a change of the temperature of 40.degree. C. The heat transfer is a function of the .alpha.-value and the temperature difference, as seen above, and therefore an increase of the speed is compensated for by a temperature difference which is smaller than the one which the change in the .alpha.-value alone indicates. A reduction of the temperature difference with 20.degree. C. can, in principle, be compensated for by an increase of speed of about 1 m/sec.
Thus, in the present invention the heat transfer from the intake air to ice/wagon is performed as quickly as or quicker than in the abovementioned prior art with a considerably lower air temperature since the higher speed results in a larger heat transfer number.
A fundamental condition for the use of a high air speed is that this speed shall be present on all the surfaces to be treated. In the apparatus according to the invention this is realized by jet streams of air or another gaseous tempering medium directed towards the object to be treated.
The advantages of using an increased air speed instead of high overtemperature are among others that the zone nearest to the wagons does not get a disagreeably high temperature. As indicated above a high temperature also results in evaporation of large quantities of water which requires considerable energy and consequently gives a high energy consumption. Through the evaporation the air gets large contents of moisture which in its turn gives rise to condensation on cold surfaces. If the air is supposed to hold a relative humidity of 30% the water contents are changed from 30 g/kg to 10 g/kg when the air temperature is decreased from about 55.degree. C. to about 35.degree. C. Translated to a cold surface subjected to condensation deposit this means that a surface temperature of about 15.degree. C. has deposit of damp for the cooler air whereas deposit of damp occurs up to a temperature of about 32.degree. C. for the warmer air.
According to an advantageous further developement of the invention circulating air is used for the deicing whereby large evaporation is avoided even if the moisture contents of the air are high.
According to another advantageous embodiment of the invention a large fraction of exterior air is used in a drying step following the thawing step whereas only a smaller part or the air is circulated. The exterior air is then preheated.
To prevent condensation in the driving motors of the trains during deicing dry air is forced to pass through the cooling air inlets of the motors. In the prior art this has been done by means of hoods which are connected to flexible tubes and placed over the cooling air inlets, with the dried air of indoor temperature being supplied through said flexible tubes during the deicing.
According to the present invention humid deicing or defrosting air is prevented from penetrating "through the backway" into the air outlets of the motors and condensing on windings and other surfaces of the motors by producing a static overpressure around the air inlets on the sides of the wagon since dynamic pressure cannot be transmitted because of the so-called labyrinth grating structure of the air inlets. The static overpressure is produced by blowing air at high rates towards the cooling air inlets of the motors, with a static overpressure being produced around the air inlet. The moisture content of the air must be lower than the dew point of the motors, and is therefore preferably formed by dry, preheated exterior air. By the thus developed static pressure the dry air is pressed through the motors and prevents humid air from penetrating through the cooling air outlets and condensing on cold surfaces in the motor.